Particulated ammonium nitrate in the form of prilled ammonium nitrate, and particularly porous prilled ammonium nitrate (PPAN) is widely used, for example in the manufacture of explosive compositions. PPAN is usually formed by prilling an aqueous ammonium nitrate solution and thereafter drying the prills. During the drying process, the water in the prills evaporates, creating the porosity in the prills to reduce density and to allow absorption of fuels during the preparation of explosives. The process of drying the prills is expensive and the drying apparatus used is also capital intensive.
In U.S. Pat. No. 5,540,793 encapsulated microspheres were introduced in the prilled product to control the density and to render the fuelled prills more sensitive. The patent also teaches that the porous prilled product so formed may include limestone in fairly large amount from 30.0% to 0.01% mass per mass final product. Some of the inventors of U.S. Pat. No. 5,540,793 and the present invention are the same and it is known to the present inventors that the limestone mentioned in U.S. Pat. No. 5,540,793 had a fairly large particle diameter of between 200 μm and 500 μm and that it was used to desensitize the product.
When limestone is added in this manner, no gas generation takes place during the formation of the prills, mainly due to the large particle size of the limestone but also due to factors such as acidity of the ammonium nitrate and the timing of the addition of the limestone.
U.S. Pat. No. 5,540,793 also teaches that the porosity of the porous prilled product which includes the encapsulated microspheres can further be enhanced by the inclusion of gas in the product during the prilling process. It is taught that this gas can be developed in situ in the products via a suitable chemical reaction. The gas may comprise carbon dioxide which is formed through decomposition of a suitable carbonate in acid medium. The patent further teaches that the carbonate may comprise any suitable water-soluble inorganic salt of carbonic acid, for example potassium and/or sodium carbonate or alternatively, it may comprise a less soluble salt.
However, it was found that it is very difficult to control the rate of the gas release and bubble size when gas is developed in situ in this manner. When the gas generation is too much the progressively formed crust, resulting from the solidification of the ammonium nitrate on the surface of the particle, will be too weak and the generated gas will destroy the crust, resulting in excessive breakage of the particles. When gas generation is too low or too slow, the required low density and increased porosity will not be developed.
It is believed the general trend is that the smaller the pores (down to about 30 μm) in PPAN the higher will be the detonation velocity, detonation pressure, and thus the brisance of an explosive composition containing such PPAN.